There are various forms of touch sensitive controls which use a capacitive sensor to sense the presence of a body such as a user's finger at a plurality of positions on the sensor. A touch sensitive capacitive sensor for example is disclosed in WO-97/23738. The capacitive sensing device disclosed in WO-97/23738 relies on measuring the capacitance of a sensing electrode to a system reference potential (earth). A single coupling plate is provided and disposed to form a touch sensitive switch. The coupling plate is referred to as a key. In accordance with this example, the key is charged using a drive circuit for a drive part of a measurement cycle and then this charge is measured by transferring the induced charge from the key to a charge measurement capacitor of a charge detection circuit during a measurement part of the cycle. Typically, a burst of measurement cycles are performed, before measuring the charge present on the charge measurement capacitor. The sensor can detect the presence of an object near the key as a result of a change in an amount of the charge induced onto the key. Effectively, this provides a measure of a change in capacitance of the key as a result of the presence of the body or object. When a pointing object for example a user's finger approaches the sensing electrode (Y plate), the pointing object appears to be a virtual ground. This serves to change the measured capacitance of the sensing electrode to ground. Thus, the change in measured capacitance is taken to indicate the presence of a pointing object. Thus, by providing a plurality of sensing electrodes, or keys, a plurality of positions on the touch sensor may be detected.
Another form of touch sensitive control is disclosed in U.S. Pat. No. 6,452,514. In this example a pair of electrodes is provided which act as a key so that the presence of a body, such as a user's finger, is detected as a result of a change in an amount of charge which is transferred between the two electrodes. With this arrangement, one of the electrodes (labeled X) is driven with a drive circuit and the other of the pair of electrodes (labeled Y) is connected to a charge sensing circuit which detects an amount of charge present on the Y plate when driven by the X plate. As disclosed in WO-OO/4400 18 several pairs of electrodes can be arranged to form a matrix of sensing areas which can provide an efficient implementation of a touch sensitive two-dimensional position sensor.
In some examples, a plurality of keys can be disposed to form a two dimensional touch sensor. Two dimensional touch sensors are typically used with devices which include touch sensitive screens or touch sensitive keyboards/keypads which are used in, for example, consumer electronic devices and domestic appliances. The two dimensional touch sensors can also be used in conjunction with an underlying display such as a Liquid Crystal Display (LCD) or a Cathode Ray Tube (CRT), to form a touch sensitive display screen. Such touch sensitive display screens have become increasingly popular and common not only in conjunction with personal computers but also in all manner of other appliances such as Personal Digital Assistants (PDAs), Point Of Sale (POS) terminals, electronic information and ticketing kiosks, kitchen appliances and the like. It is generally desirable to provide improvements in a process for detecting a body with a touch sensor employing a plurality of keys.